1. Field of the Invention
This invention is directed to a switching power amplifier circuit which generates a five level output voltage waveform to be used in high power communications transmitters. The duration of each of the levels of the output voltage is fixed to a width which minimizes generation of the third and fifth harmonics.
2. Description of the Prior Art
Prior art devices for power conversion use a full bridge as a two or three level waveform synthesis power amplifier for very low frequency-low frequency (VLF/LF) operation. The full bridge switching power amplifier has four legs (a leg being a transistor switch) with series adjacent legs being switched on alternately so that one switch is ON and the other switch is OFF producing a square wave output. Since a square wave is generated by switching between two different voltage levels, the term "two-level waveform synthesis" is used. The switches of each half of the full bridge are usually transformer driven so that the drive signal applied to one switch is exactly opposite to that applied to the other switch in the half bridge, guaranteeing that two adjacent switches are not simultaneously conducting at any point in time.
In a square wave produced by a two level synthesis power amplifier that has perfect symmetry, i.e., is exactly fifty percent (50%) ON and fifty percent (50%) OFF, even order (i.e., second, fourth, etc.) harmonics are not present. In the interest of maintaining spectral purity of the transmitted signal, it is desirable to eliminate as many of the remaining odd order harmonics as possible from the output square wave before it reaches the antenna. To attenuate the harmonics, power filters are employed in an antenna filter coupler. These power filters are typically multisection L-C ladder filters utilizing large ferrite or air-core inductors and ceramic or vacuum transmitting capacitors. This filter takes up approximately two-thirds (2/3) of the volume and weight of the entire cabinet of a power amplifier transmitter system, and is a major factor in the cost and complexity of the system.
In a standard full bridge circuit, if the drive signal to one half bridge is offset in phase with respect to the drive signal to the other half bridge, the voltage present at the output, which is the sum of two square waves, will be a waveform having a step in the middle. The voltage of the step will be intermediate between the upper and lower voltage levels. Since three voltages are present in this waveform, this waveform may be considered to have been produced through three-level waveform synthesis. By changing the phase of the drive signals to the two half bridges, the width of the output pulse is varied. A waveform produced by delaying the drive signal 60.degree. will have no third, sixth or ninth harmonic.
Another circuit proposed to deal with the harmonics in the output square wave is set forth an article entitled A High Power, High Frequency FET Inverter For A Low Frequency Transmitter in "MOSPOWER Applications Handbook", Dr. F. Lee, Dr. Dan Chen and Z. O. Fang, Siliconix Incorporated, Santa Clara, California, pages 6-139-6-145. This circuit includes a high power, high frequency power amplification technique. The technique discussed in this reference on page 6-144 employs a sine wave synthesized through, in this case, sixteen separately phased three-level power amplifiers which are transformer coupled together.
One drawback of this scheme is that it has no facility for graceful degradation. If any of the sixteen power amplifiers in the system fails, the output waveform would be distorted and the harmonic reduction of the output could not be maintained. It would be preferable to produce a high degree of waveform synthesis within each power amplifier so that when several power amplifiers are combined, the failure of a single unit would merely result in the reduction of output power and there would be no effect on the output harmonic content.
It was therefore necessary to develop a device in which at least the third harmonic (which is the odd order harmonic closest to the fundamental and is most difficult to attenuate) and the fifth harmonic can be significantly reduced and thus, the antenna coupler filter can be made smaller and less expensive by an estimated one-third to one-half that of prior art amplifiers.